The present invention relates to a signal detector for detecting specific biosignals, in particular for detecting QRS complexes or P-waves in an electrocardiogram.
The detection of specific biosignals in a disturbed signal train is a problem of high diagnostic and therapeutic relevance, e.g. in the field of cardiology and neurology. Thus, the identification of the individual segments of an electrocardiogram--the P-wave originating with the auricle, the QRS complex originating with the heart chamber and, if necessary, also the T-wave of the electrocardiogram that originates with the chamber--as well as the correct determination of the time interval between signal segments with the same origin are of central importance to the detection and successful treatment of arrhythmias, specifically for the classification of tachyarrhythmias and the correct function of pacemakers and defibrillators.
In the simplest case, the detection of the QRS complex in an ECG signal occurs through a comparison of the ECG signal with a predetermined threshold value, which is above an assumed level of signals with a different origin. If the ECG signal exceeds the threshold value, a detection signal indicating the appearance of a QRS complex is emitted.
One problem in this case is that the signal level of the ECG signal is subject to fluctuations, which can lead to false detections.
The fact that the QRS complexes can have very different amplitudes for certain arrhythmias and in the course of a pacemaker therapy is taken into account according to U.S. Pat. No. 4,940,054 by programming in three different sensitivity or threshold values as well as a preset sequence for the use of these varied threshold values. However, this solution has a very limited potential and is hardly advantageous, particularly for a fluctuating interference signal level.
A QRS detector with automatic threshold value adaptation is described in German Patent 2 805 482, for which the detection threshold for the amplitude of the R-waves follows and, after a R-wave appears, is lowered exponentially in each case with a time constant that is predetermined by the components of the analog circuit. This solution is also very inflexible, owing to the fixed time behavior on the component side.
A R-wave detector is described in U.S. Pat. No. 5,117,824, for which the detection threshold automatically follows the R-wave amplitude and in each case is lowered linearly, starting with the moment a R-wave appears until the following R-wave appears. The initial threshold value is then set again to a predetermined share of its amplitude. However, a R-wave evoked by a pacemaker pulse is ignored, and the linear lowering is continued for a predetermined time interval to be able to detect possibly following R-waves with low amplitude (such as can occur with tachyarrhythmias induced by pacemakers). However, this solution also has little adaptability to the various use/interference signal constellations.
The signal detector shown in U.S. Pat. No. 5,339,820 has a refractory interval, and the detection sensitivity is adjusted automatically. If a predetermined initial detection threshold (detection event) is exceeded, the signal detector starts a refractory interval, during which no further detection events can occur, so as to prevent that an individual QRS complex is detected falsely more than once. In addition, the signal detector determines the amplitude for the QRS complex during the refractory interval. The threshold value is subsequently set to a new value (preferably 75% of the amplitude) in dependence on the previously determined QRS complex amplitude. Subsequently, the threshold value is lowered step-by-step with a timing control. The lowering of the threshold value stops if either the following QRS complex is detected or the threshold value has reached a preprogrammed lower limit value.
However, the input signal fluctuations often do not or not only impact the specific useful signal, but (also) other signals, among other things interference signals. In such a case, it can occur that the known signal detector lowers the threshold value below the level of the nonspecific signals, which leads to false detections. In addition, a method which provides exclusively for the lowering of the detection threshold is not sufficiently differentiated for a detection of specific signals in complex signal sequences.
It is therefore the object of the invention to create a signal detector of the aforementioned type, which permits an automatic adjustment of the detection sensitivity or amplification to avoid false detections for a fluctuating level of the useful signal as well as the interference signal segments and which also allows a detection of specific useful signals within complex signals.